Project Summary Melanoma, a cancer that can be surgically cured when detected early, can be difficult to distinguish from common, benign pigmented spots (nevi) that develop on human skin, a situation that leads to the unnecessary excision of benign nevi and the failure to detect some evolving melanomas. Intriguingly, the most frequent oncogenic mutation in melanoma?activation of BRAF?is also the cause of most nevi. Yet, unlike melanoma, nevi spontaneously growth-arrest. The explanation has been attributed to ?oncogene-induced senescence?, but a variety of observations indicate that what holds nevi in check is not cell-autonomous senescence, but rather reversible interactions among nevus cells, and between those cells and their environment. Such interactions resemble the intra-lineage feedback mechanisms used by many normal tissues to achieve growth control and size homeostasis. Using an inducible Braf-mutant mouse model, we recently made the observation that nevi are not only composed of pigmented cells but also a novel melanocyte population that forms a ?veil? around the pigmented cells, consistent with the existence of lineage relationships that could underlie such a feedback strategy. In this project, we will investigate the role of these cell types in mouse models that produce both nevi and melanoma, seeking to identify both the nature of growth control and the means by which melanoma cells escape from it. We will integrate mathematical modeling to develop hypotheses that can explain the spatiotemporal dynamics and spatial statistics of nevus and melanoma development in these models, including potential bifurcations that account for the development of both nevi and melanoma in the same mouse. We will use time-course single cell RNA-sequencing to identify potential positive and negative feedback regulators that drive such models, and will carry out experiments to test model-based predictions concerning the roles that such molecules play. Finally, we will investigate the phenomenon of spontaneous regression, which occurs with both mouse and human nevi, for clues into how the immune system efficiently recognizes melanocyte overgrowth. Such information will be useful in developing new prevention and therapeutic strategies for this devastating disease.